<h2>DESCRIPTION</h2>

<em>r.topidx</em> creates topographic index (wetness index), ln(a/tan(beta)), map from
elevation map
<p>where
<dl>
<dd>a: the area of the hillslope per unit contour length that drains through any point,
<p><dd>tan(beta): the local surface topographic slope (delta vertical) / (delta horizontal).
</dl>
<p>Input maps may have NULL values.  For example, if you have a MASK for
a watershed (basin map from <em>r.water.outlet</em>), the
following command will create a masked elevation map (belev):
<div class="code"><pre>
r.mapcalc "belev = if(isnull(basin), basin, elev)"
</pre></div>
<p>
<em>r.stats -Anc</em> prints out averaged statistics for topographic index.

<h2>EXAMPLE</h2>

Running the command below will generate the topographic index map shown in the figure:

<div class="code"><pre>
g.region raster=elevation -p
r.topidx input=elevation output=topidx_elevation
</pre></div>

<div align="center" style="margin: 10px">
<img src="r_topidx.png" alt="r.topidx example" border="0">
<br>
<i>Figure: topographic index map derived from elevation map</i>
</div>

<h2>SEE ALSO</h2>
<em>
<a href="r.topmodel.html">r.topmodel</a>,
<a href="r.water.outlet.html">r.water.outlet</a>,
<a href="r.mapcalc.html">r.mapcalc</a>
</em>
<br>
<a href="http://idea.isnew.info/r.topidx.html">Known issues</a>

<h2>REFERENCES</h2>

Cho, H., 2000. GIS Hydrological Modeling System by Using Programming Interface
of GRASS. Master's Thesis, Department of Civil Engineering, Kyungpook National
University, Korea.
<p>
Moore, I.D., R.B. Grayson, and A.R. Ladson, 1991. Digital Terrain Modeling: A
Review of Hydrological, Geomorphological, and Biological Applications.
Hydrological Processes 5:3-30.

<h2>AUTHORS</h2>

<a href="mailto:grass4u@gmail com">Huidae Cho</a><br>
Hydro Laboratory, Kyungpook National University, South Korea
<p>
Based on GRIDATB.FOR by Keith Beven.

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